Traditionally, strings for musical instruments have been made of natural gut derived from the intestines of animals, or metals such as steel, or synthetic materials particularly polyamide 6 or polyamide 6.6 and their copolymers, known generally as nylon. Strings may be simply single monofilaments of these materials, or they may consist of a core around which is wrapped metal wire or other material to increase the mass of the string without substantially increasing its lateral stiffness.
Natural gut is fragile and is greatly affected by changes in the ambient humidity, which can lead to the need for frequent retuning by the player. As a natural product, it is inconsistent in properties and deteriorates rapidly under adverse conditions of temperature and humidity. Nevertheless, some musicians believe it produces a superior sound, which favours its use on bowed instruments, although because of its deficiencies its use on classical guitars has been completely superseded by nylon.
Nylon has gained wide acceptance as a substitute for natural gut in music strings: it has the advantage of being a consistent manufactured product and is considerably more durable than natural gut. However, it still has certain deficiencies, notably a sensitivity to changes in humidity and significant loss of tension with time, so that retuning is often necessary. It also has a higher degree of internal damping than is desirable. In particular, a low degree of internal damping is required for a string used on a guitar or other plucked instrument, otherwise the vibrations of the string decay too quickly and the sound is dull and lifeless.
Some attempts have been made to reduce the internal damping of nylon music strings by various treatments. For example, U.S. Pat. No. 3,842,705 describes the use of irradiation by high intensity ionising radiation to improve the playing quality of nylon strings, and U.S. Pat. No. 4,015,133 similarly describes the use of radiation to improve the elasticity and reduce the damping in polyamide strings. However, these treatments require the use of radioactive sources or high intensity electron beams, and are expensive and technically difficult to carry out.
Steel strings do not suffer from the effects of humidity but their use is not generally acceptable on the classical guitar since they are necessarily much thinner than nylon or gut strings, which leads to difficulties in plucking with the fingers and reduces the control which the player has over the tone quality of the sound.
Other materials have been suggested for use in music strings. For example, U.S. Pat. No. 4,833,027 describes the use of polyvinylidene fluoride for music strings. European Patent 49368 discloses a string made of polyvinylidene fluoride and acrylate copolymer. Japanese Patent 61114297 describes music strings made of drawn polyacetal. However, none of these materials has shown in practice any substantial advantage over nylon strings in terms of stability or internal damping. Nylon string products dominate the guitar string market both as monofils and as wrapped multifils.
The objective of the present invention is an improved music string which may have significantly lower internal damping than presently available strings, may show low inhamonicity, and may be unaffected by ambient humidity changes. The invention may be understood in temps of the following theory, but is not dependent on the correctness of the theory and is not intended to be limited by it.
It is well known (see for example "The Theory of Sound" Volume I Section 189 by Lord Rayleigh published by McMillan & Co. 1984) that a vibrating string produces not only a fundamental frequency but also a series of hamonics of higher frequency than the fundamental. The frequency .intg..sub.n of the nth harmonic is given by: ##EQU1## where:
l=length of vibrating string PA0 T=tension in the string PA0 m=mass per unit length of the string PA0 r=radius of the cross-section of the string PA0 E=Young's modulus of the string material
As n gets larger, the harmonics of the string deviate more and more from a simple whole number ratio with each other, leading to dissonance and an unsatisfactory quality of sound.
In order to minimise this effect, the second term in the bracket in the above expression must be as small as possible. That is to say: ##EQU2## must be small. I is referred to as the Inharmonicity Factor of the string. I may be rewritten as: ##EQU3## where .rho.=density of the string material and the other symbols have the same meaning as before. Thus to minimise I, the value of E/.rho..sup.2 for the string material must be small.
It is found (see for example J. C. Schelling: Journal of the Acoustical Society of America Volume 53 (1973) Pages 26-41) that generally the value of I should not exceed about 6.times.10.sup.-5 for satisfactory string performance, and preferably should be much less.
Some values of E/.rho..sup.2 for known materials are given in Table I, together with the corresponding value of I when the material is used as a monofilament for the 3rd or G string of a classical guitar. Although monofilaments of nylon and steel give acceptable levels of inharmonicity for use as the highest three strings of a guitar, it can be seen that polyethylene terephthalate and aluminium, for example, are unacceptable.
TABLE I ______________________________________ Inharmonicity factor I for guitar 3rd strings made from different materials Material E/.rho..sup.2 .times. 10.sup.-3 I .times. 10.sup.5 ______________________________________ Natural gut 0.6-2.08 0.76-2.64 Nylon 2.56 3.25 Steel 3.45 4.37 Polyester (PET) 7.19 9.12 Aluminium 9.67 12.26 ______________________________________
In order for a string material to be acceptable for use as the highest three strings of a guitar, the value of E/.rho..sup.2 should not exceed about 5.times.10.sup.3 and preferably should be less than 3.times.10.sup.3. However, it must be understood that although a low value of E/.rho..sup.2 is a necessary condition for low inharmonicity in a string, it is not in itself sufficient, since inharmonicity depends on other parameters including the length and tension of the string.
The motion of a vibrating string is damped by both viscous interaction with the surrounding air and by visco-elastic mechanisms within the string material itself. Both damping mechanisms lead to an exponential decay in the amplitude of vibration of a plucked string from the moment it is first set in motion. The amplitude of the vibration at time t is given by: EQU A.sub.t =A.sub.o e.sup.-t/.tau. (iv)
In this expression, A.sub.o is the initial amplitude of the string vibration, e is the base of the natural logarithms, and .tau. is the characteristic decay time, i.e. the time required for the amplitude of vibration to decay to 1/e of its initial value.
The decay time .tau. observed for a given string is a combination of the decay time for air damping (.tau..sub..alpha.) and internal damping (.tau..sub..alpha.). Since both mechanisms act simultaneously, the total decay time .tau. is given by: ##EQU4##
The air damping of a string is dependent only on the mass and diameter of the string, and not on the elastic properties of the string material itself
Thus: EQU .tau..sub..alpha. .varies..rho.d .intg..sup.-1/2 (vi)
where d is the string diameter.
On the other hand, the internal damping is due to the inherent properties of the string material. All real materials, especially polymeric materials such as natural gut and nylon, show a time dependence of strain on applied stress. This so-called visco-elastic behaviour means that when a stress is applied the final value of strain is not achieved instantaneously, but requires time to reach its equilibrium value. This type of behaviour can be represented by expressing the Young's modulus of the material as a complex number, i.e. EQU E=E.sub.1 +iE.sub.2 (vii)
where E.sub.1 is the modulus contributed by normal elastic bond distortion, and E.sub.2 is the contribution from bond rotation and movement of kinks in the polymer chains. The decay time due to this mechanism can be shown to be represented by: ##EQU5##
A more extended account of the damping of music strings has been given by N. H. Fletcher in "Paper given to the Catgut Acoustical Society Technical Conference, Montclair, N.J., April 1975" published by that Society.
The visco-elastic behaviour of natural and synthetic polymers such as natural gut and nylon is well known, so it is not surprising that the damping of strings made from these materials is high. Metals show lower visco-elasticity and correspondingly lower damping when used in music strings.
Since the decay time for internal damping is proportional to the inverse of frequency, the decay of the higher harmonics of the string relative to the fundamental will be greater than would be the case if only air damping applied since air damping is proportional only to the inverse square root of frequency. Hence a reduction in internal damping will lead to more sustained higher harmonics, and therefore a brighter and more lively sound. In particular, the enhancement of the second harmonic is well known to produce a more brilliant tone.
We have now found that, surprisingly in view of their polymeric nature, aromatic polyetherketones can be processed into strings in such a way that the internal damping is reduced to a very low level. In addition, these strings may have a value of E/.rho..sup.2 which is less than 5.times.10.sup.3 and generally less than 3.times.10.sup.3.
Polyetherketones have the general formula. EQU --Ar--O--
where Ar is an aromatic radical and at least some of the Ar radicals contain a ketone linkage. A preferred thermoplastic aromatic polyetherketone is polyetheretherketone, having the repeat unit: EQU --O--Ph--O--Ph--CO--Ph--
where Ph is a p-phenylene. This polymer can be readily melt spun and drawn into filaments which we have found to show remarkable durability and stability under tension, and are virtually unaffected by ambient humidity.
The superior damping properties of strings of the present invention may be characterised by measuring the decay time of the fifth harmonic of the string when it is set into vibration by plucking, and comparing this with the decay time of the fundamental vibration of the string. The ratio of the decay time of the fifth harmonic to the decay time of the fundamental is called the Damping Ratio.